A receiver of radio frequency signals, for example a radio receiver or a television (TV) set, is usually characterized by two primary components. One being that of a tuner for selecting a frequency or band of frequencies intended for reception and the second being for processing the selected signals whereby information is presented to a user in an intended format. Some formats are radio telemetry, radiotelephone and broadcast radio programming, the later two usually being audibly reproduced. Another format is that of television and television programming. Television is characterized by a visible presentation of visual information signals. In television programming the visible presentation is usually accompanied by an audible reproduction of audio information. Over the years tuners have been provided which operate by various different methods for selecting a frequency band, or channel, for reception. Some examples of methods of operation include, tuned radio frequency, super regenerative, and superheterodyne.
A radio receiver operating by the tuned radio frequency method includes a series of variable bandpass filters and amplifiers. The filters are tuned in tandem to amplify a desired signal frequency band more than other signal frequencies. In the early days of vacuum tube radios the tuned radio frequency method provided unsurpassed fidelity of audio loudspeaker reproduction and it was the preferred method of selecting a radio program signal. However, such tuners generally had limited sensitivity and selectivity and were comparatively expensive. Presently loudspeaker radio receivers operated by the tuned radio frequency method are rare and might be considered to be collectibles.
A radio receiver operating by the super regenerative method uses feedback and includes fewer devices. It could provide very sensitive and selective reception but reproduction of the audio signal could include annoying noises such as hissing and whistling, induced by the tuner's operation. Its operation often required some skill on the part of the user to get satisfactory reception and reproduction of the radio program. Consequently, in spite of its lesser expense, it was never really found to be favourable. Presently, radio receivers operating by the super regenerative method are very rare.
Receivers that operate using the superheterodyne method are commonplace. A superheterodyne receiver converts a desired signal to an intermediate frequency (IF) for filtering using a fixed bandpass filter. Signals having been passed through the fixed bandpass filter are processed by a second primary component of the receiver. A fixed bandpass filter is preferred because the filter characteristics are more readily and precisely determined and hence the desired signal is more readily distinguished from noise and other unwanted signals. Surface acoustic wave (SAW) filters are exemplary of the state of the art fixed bandpass filters.
More specifically, the tuner operates by mixing received signals with a locally generated oscillator signal to generate sum and difference signals. The user adjusts the local oscillator so that a predetermined one of either the sum or the difference signals are within the passband of the filter. This process is referred to as frequency conversion, where a signal of the desired frequency of reception is converted to the IF signal. IF signals passed by the filter are either directly supplied to the second primary component of the receiver, or a first converted to base band and then supplied to the second primary component of the receiver. If the receiver is required to provide superior selectivity, then more than one intermediate frequency, corresponding mixers, and IF filters are used. Also, improved sensitivity is obtained if a low noise amplifier and a filter are used to amplify received signals with emphasis on signals in a band of interest before the signals are converted by the mixer to the IF signals. In a receiver intended to receive commercial television signals the band of interest lies between about 50 MHz and more than 800 MHz.
The tuner element in any radio receiver, or TV set, is critical to the satisfactory quality of presentation of received program information. The tuner must accept the full bandwidth of a desired signal frequency to the substantial exclusion of any other signals. The tuner must operate in a linear fashion over a wide range of signal strengths. The tuner must not generate any significant noise, phase distortion or image frequency signals which would be deleterious to the eventual presentation of the program information to the user. The typical manufacturing cost of a quality TV tuner in relation to the manufacturing cost of an entire TV set is very substantial in spite of ongoing evolution of tuner design.
At one time, the standard TV set was limited to receiving only 12 channels within the very high frequency (VHF) spectrum, five channels (2–6) in a lower band and seven channels (7–13) in an upper band. A television tuner typically included an amplifier, an oscillator and a mixer, which were selectively coupled with one of 12 units, each having an input filter and a corresponding oscillator tank circuit. Each unit was dedicated for use in receiving a predetermined one of the 12 channels. Each unit was carried on an elongated insulating substrate and arranged in combination upon a rotatable framework to form a turret like cylinder. This so called “turret tuner” was rotated by the user to any of the 12 positions to effect connection, via gold plated electrical contacts, of the appropriate unit with the oscillator amplifier and the mixer to receive the desired TV channel. After manufacture, each unit required manual adjustment in order to be optimized for receiving the predetermined channel and included at least a variable inductor for this purpose. A fine-tuning control for user adjustment was required for each unit so that acceptable reception of a selected TV program would be available.
The TV turret portion in TV tuners was almost always a separate or modular element. This was convenient for manufacture and facilitated easy replacement by a service technician, when the plated contacts became worn. However, the amplifier, oscillator, mixer, as well as the fixed bandpass filter, often referred to as an IF strip, were more or less integral with the TV set.
Broadcast television tuners for TV sets have gone through a design evolution over a period of more than 60 years. The earliest tuners utilized vacuum tube technology and required that a minimum number of vacuum tubes be used due to their cost, power consumption, heat generation, and bulk. Therefore, passive components, such as resistors, capacitors, inductors and transformers, were used as much as possible in most designs. This style of design using vacuum tubes continued until about 1960. Thereafter TV tuner components, particularly vacuum tubes, began to be replaced by transistors. In the early 1970's many functions of the tuner utilizing only one tube or transistor were replaced with 4 to 20 individual transistors, which in combination could perform the same function with better precision, less space, less power, less heat generation and lower cost. Bipolar transistors were popular until sufficiently stable metal oxide silicon field effect transistors (MOSFETs) became available in the late 1970s. However, the active device count still defines the cost and size limits of TV tuners and active device count minimization is continued.
The introduction of the integrated circuit was gradual, firstly limited to only lower frequency functions and eventually, as the integrated circuit MOSFET based technology evolved, use for higher frequency functions became practical. Nonetheless, many comparatively expensive passive discrete elements remained in TV tuner designs. In contrast to historical tuner designs, where passive elements were of lower cost, presently the cost of accommodating a few non-integrated off-chip passive elements, in combination with an integrated circuit in a TV tuner, has become a significant part of the cost of the tuner.
Recently available consumer electronic based devices, other than TV sets, may also be equipped with a TV tuner. For example within the last 20 years, video cassette recorders (VCRs) have been provided with an air and cable ready tuner. TV tuners are now typically modular in nature, that is not a built-in part of a TV set, or VCR, but an entirely modular, or separately plugged-in, element. A modular tuner may be arranged to provide an output signal at the customary 43 MHz intermediate frequency, or in any of several baseband signal formats, including separate audio and video outputs. This facilitates easy replacement if the tuner should fail as for example may occur in an event of a nearby lightening strike. When a tuner is used in a typical TV set with a cathode ray tube (CRT) display, the tuner size is not critical because the television set has an inherently large size, but when a tuner is used in other electronic equipment, or in association with a flat screen display device, space is at a greater premium. TV tuners are now being used in smaller and smaller computers, television sets, and VCRs. In recent years, some TV tuners have been reduced in size to modular units that are as small as about ½″×2″×⅜″. As the equipment in which tuners are utilized becomes smaller, it is preferred that the bulk of the TV tuner must also at least decrease proportionately. In one example the modular TV tuner is manufactured to predetermined electronic and physical specifications for plug-in installation into a personal computer (PC) system or a laptop PC. This allows the PC to have the additional function of a TV set, however at the present time a retail price of such a modular tuner can be as much as a third or more of the retail price of the PC itself.
A presently manufactured TV tuner typically comprises one or two integrated circuits and numerous discrete elements being inductors, capacitors and transistors. IF to baseband conversion typically includes another integrated circuit, several filter elements, tuning and control elements, such as resistors and potentiometers, variable inductors and capacitors, and may require some other ancillary external components. There may be more than 100 elements on a presently manufactured tuner circuit board. Some of these elements are of course directly connected with one of the integrated circuit elements, where each such connection adds substantially to the cost of the integrated circuit tuner. Furthermore, state-of-the-art TV tuners usually require that each tuner be manually adjusted before leaving the factory. Comparative to the manufacturing cost of a TV set or VCR, the cost of the modular TV tuner portion thereof remains significant.
One advance in filter technology, the surface acoustic wave (SAW) filter, brought a significant change in that some discrete filter components such as capacitors and manually tuned inductors can be avoided or at least reduced in number. Filtering performance is improved and the resulting tuner design requires less space and is somewhat less costly. However, the SAW filter, which is fabricated on a ceramic substrate, is an off-chip device. It is also a rather low impedance device. Its low impedance matching, as well as drive requirements, results in complications such as significant power consumption. Furthermore broadband circuits tend to consume more power as compared to narrower band circuits. Consequently, as the upper frequency for receivable TV signals increases the power consumption of broadband amplifiers increases, particularly when used in combination with a SAW filter or filters. Heat dissipation and heat concentration in the already reduced surface area of a small TV tuner, adds heat stress to the circuit components therein as well as to nearby elements of the electronic apparatus. The consequent heat stress thus unfavorably affects the functional reliability of both the tuner and any nearby elements.
A need therefore exists to provide an integrated tuner circuit that requires less space, and is somewhat less costly than existing tuners, while offering improved performance and reduced power consumption over prior art designs.